In recent years, with the increased demand for bulk data transmission, large-scale networks using a high-speed digital transmission method has been constructed. The use of the Optical Transport Network (OTN), which is prescribed by International Telecommunication Union Telecommunication standardization sector (ITU-T) Recommendation G.709 as the digital transmission standards, is being studied. The OTN is used to transmit a frame that is a signal in which a signal (overhead) used for maintaining a transmission apparatus or the like is attached to a digitized main signal (pay load).
An example of a method for constructing a large-scale network using the OTN includes a Bi-directional Line Switched Ring (BLSR) method in which, for example, transmission apparatuses are connected in a ring shaped fashion. Some ring networks constructed using the BLSR method uses an automatic protection switching (APS) function for recovering a network when a failure occurs. In the ring network that uses the BLSR method using the APS function (hereinafter, referred to as an “APS/BLSR ring network”), high-speed failure recovery is implemented by transmission apparatuses notifying each other of APS bytes stored in the overhead contained in a frame. For example, in the APS/BLSR ring network, when a signal fail (SF) occurs as a failure, a network is recovered by notifying an adjacent apparatus with information on the network's own apparatus to by using the APS bytes containing the SF and by switching the currently used line to a spare line.
When constructing a large-scale network that is larger than the above, it is preferable to implement a multi-ring configuration in which multiple ring networks are connected with each other. Such a multi-ring configuration as that illustrated in FIG. 11 has been proposed. FIG. 11 is a schematic diagram illustrating a conventional multi-ring configuration. As illustrated in FIG. 11, with the conventional multi-ring configuration, two transmission apparatuses in a ring network A are connected with two transmission apparatuses in a ring network B, respectively.    Patent Document 1: Japanese Laid-open Patent Publication No. 2003-18891    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-266480    Patent Document 3: Japanese Laid-open Patent Publication No. 2003-101559    Patent Document 4: Japanese Laid-open Patent Publication No. 2002-232442